Lunatia lewisii (previously known as Polinices lewisii), common name Lewis's moon snail, is a species of large operculated sea snail. It is a predatory marine gastropod in the family Naticidae, the moon snails. 
This is the largest species in the family.
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The shell of this species can grow to 14 centimetres (5.5 in) across, the largest of the moon snails. It has an extremely large foot, which when the snail is active, is extended up over the shell and mantle cavity. Part of the propodium contains a black-tipped siphon which leads water into the mantle cavity. The cephalic tentacles, located on its head, are usually visible above the propodium.
When the animal retracts its soft parts into the shell, a lot of water is expelled, thus it is possible to close the shell with its tight-fitting operculum.
Like other moon snails, this species lays its eggs in a "sand collar". The eggs may number in the thousands and hatch into microscopic larvae which feed on plankton until they undergo torsion and metamorphose the adult stage.
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